In a range of different common medical disorders (among them panic anxiety, epilepsy, febrile seizures, asthma, idiopathic dyspnea, and certain heart conditions) patients are often found to have a lowered carbon dioxide tension in the body. This condition is called hypocapnia and is in almost every case caused by hyperventilation, meaning that the patient is—on average—breathing in excess of metabolic requirements, a reaction that may be initially prompted by emotional or physical stress. Hyper-ventilatory breathing patterns may be characterized by either sudden hyperventilation attacks, slight chronic overbreathing, oscillations between over—and underbreathing, and/or a generally normal breathing rate interspersed with frequent sighs and/or deep breaths. In the case of sudden increases in the ventilatory flow, one of the most important consequences is an almost instantaneous drop in the arterial carbon dioxide tension, followed rapidly by a rise in the pH value of the blood (alkalosis), in turn affecting the blood flow to the brain, the oxygen delivery to the cells, the excitability of the nervous systems, the tension of the smooth muscles, as well as other physiological parameters. These changes can lead to a range of symptoms depending on the individual patient, including dyspnea, chest pains, panic, tingling/numbness in the extremities, cramps, fainting, seizures etc. Often, these symptoms will be ascribed to other afflictions, such as a heart condition, asthma, epilepsy, panic anxiety etc., where in fact the symptoms are induced by hyperventilation.
If hypocapnia is sustained over days or weeks, the kidneys will gradually compensate for the rise in the pH value. This process is called compensatory metabolic acidosis and will partially (but rarely completely) restore the normal blood pH value. Many other parameters, among them arterial CO2 tension, bicarbonate concentration and other ion concentrations, will remain at abnormal levels, however. In particular, the reduction in the bicarbonate concentration will impair the acid/base buffering capacity of the body.
Long-term hyperventilation may also have a self-sustaining effect. Under normal circumstances, the alkalosis induced by hyperventilation will diminish the chemoreceptor output signal, thereby lowering the ventilation, i.e. constraining hyperventilation. However, this physiological “brake” is weakened by the compensatory metabolic acidemia which develops in long term (days or weeks) hyperventilation. It is possible that this can lead to hyperventilation becoming self-sustaining.
Furthermore, the theory has been proposed that hyperventilation and hypocapnia can be due to a heightened carbon dioxide sensitivity of the chemoreceptors and/or the respiratory center in the brain, possibly as a result of long term hyperventilation inducing a change in body homeostasis. This would indicate a change in the ventilatory “set point” which can be hard to reverse.
Referring to the prior art, a rebreathing mask is disclosed in PCT patent application WO97/28837, consisting of a rim section to which is attached a bag made of a material impenetrable to the passage of air. U.S. Pat. No. 4,508,116 discloses a rebreathing apparatus, in which the user breathes through an elongated air pathway from the atmosphere to the mouthpiece, achieving an accumulation of carbon dioxide. U.S. Pat. No. 3,513,843 discloses a respiratory device for regulating the carbon dioxide level of inhalation consisting of a nose-mouth mask connected to an inflatable sack; of readily variable size to adjust the same to the breathing capacity of the user, said sack having an adjustable two-way flow breather valve and a one-way outlet valve. U.S. Pat. No. 4,275,722 discloses a breathing apparatus in which a mouthpiece and a valve device between the mouthpiece and the chambers for directing the air flow are provided, and an adjustable mechanism is used to vary the amount of exhaled air which is mixed with ambient air to be rebreathed. U.S. Pat. No. 5,647,345 discloses a breathing apparatus comprising a mixing chamber with a breathing port and at least one vent port. UK patent number 2378904 discloses a breathing apparatus comprising an enclosed bag which is impermeable to moisture, the bag being fitted with a mouthpiece adapted to be held between the user's lips, the mouthpiece being in fluid communication with the interior of the bag, and wherein a filter is interposed between said mouthpiece and said bag and wherein there is provided a fresh air inlet between said mouthpiece and said filter. U.S. Pat. No. 4,192,301 discloses a breathing apparatus consisting of a disposable, flexible polymer bag which attaches to a nose/mouth mask and an air control valve located between the mask and the disposable bag which adjusts the ratio of rebreathed to fresh air through a fresh air inlet. The fresh air inlet has a check valve preventing exhaling air therethrough and a pressure relief valve may be provided to release rebreathed air when the polymer bag becomes filled. Ukranian patent number UA 74957 C2 discloses a breathing mask fitted with an expandable bag and two adjustable valves for allowing fresh air to flow into the mask volume. German patent number DE 19912337 discloses a breathing mask fitted with a tube, connecting the mask to an expandable bag; at the connection between the tube and mask, an opening of adjustable size is fitted.